She doesn't blame Mizora for it, that's something Karlach does. Karlach specifically says that she understands how devils are with their tricks. Karlach is also truly disappointed in you and you have to be very careful with dialogue choices to not have her break up with you. That's what I at the very least expected from Shadowheart as well. But all she does is say she understands and even if you pick the most brutal dialogue options indicating that you're more or less planning on doing it again, she just reacts sarcastically.
The "swan" stuff is where she essentially says she expected you to cheat on her, because if she didn't want to get cheated on she'd have gotten together with a swan. It's ridiculous.
And yes, at the very least there should be a massive approval drop. But that's the case for all companions. Approval just feels more or less abandoned as a system in act 3.
It has nothing to do "expecting you to cheat", its more of "I understand that other ppl are attractive too, but we are in relationship, so please ask me about it next time". Yes it's shitty and I dont like it too, however they'll need to rework whole conversation if changes are to be made. I don't think Larian will do that. Thats why I talk about just adding disapproval, because it makes sense. P.S. I won't cheat on her anyway, so I'll stay as pure as swan.
